CRM evaluation and preliminary environmental assessment in an abandoned silver mining district: the case of Monte Narba mine, SE Sardinia

The domestic supply of Critical and Strategic Raw Materials (CRM, SRM) is a current challenge for EU due to its strong reliance on imports. The implementation of recycling is one of the main strategies addressed by the 2024 EU Critical Raw Materials Act for the recovery of CRM and SRM within EU, and mining waste is a major potential secondary resource of such raw materials. In EU, Extractive Waste, represents the largest waste stream in the EU, with an estimated annual production of 600 Mt (NEMO Horizon, 2020), added to the historic stockpiles (28,000 Mt). This study focuses on the abandoned Monte Narba mine, historically the most important for the exploitation of hydrothermal Ag-bearing mineralization associated with Pb, Zn, F, Ba and other metals in the Ag SE Sardinia Mining District (Naitza et al., 2019). Mining activity began in the second half of the 19th century, with production reaching approximately 1,000 t/y of Ag and Pb minerals. Recently, the site attracted interest due to the presence of industrial minerals such as barite and fluorspar in the mine wastes, historically considered waste but today classified as CRM and potentially suitable for recovery. However, no valorisation of the site has ever taken place, and today the area remains abandoned, with information coming from few historical documents (Mezzolani & Simoncini, 1989). Monte Narba mine wastes cover an area of approximately 51,000 m² and an estimated volume of about 103,000 m³ (RAS, 2003). Two main waste bodies can be distinguished: a main stockpile of gravity tailings (42,000 m2), located near the old processing plant and a more heterogeneous landfill (Rio Figu), composed of both waste rock and fine residues (9,000 m2). Results of geochemical analyses indicate the presence of significant concentrations in mine wastes of Ag (average 596 ppm), both in the gravimetric tailings and in the Rio Figu waste dump. The central part of the tailings dump showed strong enrichments in CRM such as fluorite (up to 18 wt%) and barite (0.2-0.3 wt%), together with Pb (up to 1.2 wt%) and Zn (up to 1.9 wt%). The presence of fluorite is also confirmed by XRD and SEM-EDS investigation, which also reveals some phosphate phases associated with LREEs such as La (44 ppm) and Ce (85 ppm). The results indicate that, in addition to Ag, the possibility of recovering associated by-products could be considered. A preliminary environmental assessment of the site was carried out through ABA and SPLP tests on samples from both the fine tailings and the Rio Figu dumps. Overall, the results indicate that most of the samples show neutral to slightly alkaline pH conditions (pH 7-8), with negative or slightly positive AMD Net Acid Potential Production. However, the concentrations of PTEs (including Pb, Zn, Cd, As and Sb) in the raw waste always exceed the Italian contamination threshold values for industrial soils, highlighting a potential environmental concern due to their release in waters.